The invention relates to modifying the metal ion sorption capacity of a medium.
Water from waste streams, ground water, holding ponds, water treatment facilities, storage tanks, rivers, and streams can contain metals such as iron, zinc, cesium, plutonium, strontium, technetium, uranium, and americium. For environmental compliance, it is often desirable or necessary to remove these metals from the water.
A variety of methods have been developed for removing metals from the water in waste streams, ground water, holding ponds, water treatment facilities, storage tanks, rivers, and streams. Some of these methods include passing the water containing through a medium that removes the metal. The medium may be an ion exchange medium that is capable of sorbing the metal ions in the liquid. The medium is often packed in a column and once the medium is saturated with metal ions, the medium and/or column is discarded.
A method for removing metals from the medium involves eluting the metal ions from the medium with a strong acid followed by regenerating the medium with a strong base. These methods, however, do not always perform with the same level of effectiveness for all metals. For example, ion exchange media used for the removal of strontium frequently have a relatively low capacity for strontium due to large excesses of calcium and magnesium, which compete with the strontium for sites on the medium. Large excesses of calcium and magnesium relative to strontium are often present in waste streams and ground water.
A variety of agents can be used to elute metal ions from an ion exchange medium. Nitric acid and hydrochloric acid, for example, are often used to elute strontium from a strontium absorber. Nitric acid and hydrochloric acid, however, tend to cause a gradual increase in back pressure in systems in which they are employed as the eluant and in systems in which the medium is reconditioned.
The invention features a process for modifying a medium to increase its capacity to sorb (i.e., adsorb, absorb and combinations thereof) metal ions, as well as processes for regenerating the metal ion sorption capacity of a medium that has been exposed to metal ions, as well as the modified media, itself.
In one aspect, the invention features a process for modifying a medium that includes treating a medium having a metal ion sorption capacity with a solution that includes (a) an agent capable of forming a complex with metal ions, and (b) ions selected from the group consisting of sodium ions, potassium ions, magnesium ions or a combination thereof, to create a medium having an increased capacity to sorb metal ions relative to the untreated medium.
In preferred embodiments, the complexing agent is an organic acid (e.g., citric acid) and the ions are sodium ions. In some embodiments, the solution includes sodium azide. In other embodiments, the solution includes an organic acid and sodium hydroxide.
In preferred embodiments, the treating solution has a pH of between about 6 and 10, more preferably a pH of between about 7.5 and 8.5.
In one embodiment, the medium is capable of sorbing strontium ions. In other embodiments the medium is capable of sorbing mercury ions.
In another embodiment, the medium includes a membrane filled with particles, e.g., particles selected from the group consisting of particles of sodium titanate (i.e., sodium titanate, sodium nonatitanate, and combinations thereof), crystalline silico titanate, mixed salts of titanium silicate, sulfonated styrene divinyl benzene, SAMMS self-assembled monolayers on mesoporous supports specific for mercury analytes having a formula SiO2xe2x80x94CH2CH2xe2x80x94SH, and combinations thereof.
In other embodiments, the medium includes sorbed metal ions.
In one embodiment, the process further includes contacting the treated medium with a liquid that includes metal ions such that the metal ions sorb onto the medium. The medium that includes sorbed metal ions can then be treated with an agent capable of forming a complex with metal ions for a period sufficient to elute the metal ions. One example of an agent capable of forming a complex with metal ions is a solution that includes an organic acid, e.g., citric acid and sodium hydroxide.
In one aspect, the invention features a process in which the back pressure produced during the process remains relatively constant during the process. In one embodiment, the process further includes providing a medium that includes sorbed metal ions, prior to treating the medium.
The process is useful for treating a medium (e.g., a solid phase ion exchange medium) that has sorbed metal ions (e.g., heavy metals, rare earth metals, and radioactive elements). The processes can regenerate (i.e., restore or increase) the metal ion sorption capacity of articles that have been previously contacted with a source of metal ions. The process is particularly useful in regenerating the ion sorption capacity of articles that are used to remove metal ion contaminates, and to treat aqueous streams from sources such as ground water, storage tanks, holding ponds, waste water treatment facilities, and nuclear waste storage tanks.
The process of the present invention improves the metal ion sorption capacity of an article relative to its metal ion adsorption capacity without treatment. In another embodiment, the process of the invention improves the metal ion sorption capacity of an article that includes sorbed metal ions. The processes according to the present invention also permit the maintenance of a relatively constant back pressure throughout the process.
Certain preferred processes according to the present invention are particularly well suited and can be optimized for the selective removal and recovery of strontium from a medium.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
The process includes treating a medium having a metal ion adsorption capacity with a solution that includes: A) an agent capable of forming a complex with at least one metal ion; and B) ions selected from the group consisting of sodium, potassium, magnesium and combinations thereof, to increase the capacity of the medium to sorb metal ions relative to the untreated medium.
The treating solution is a buffer preferably having a pH in the range of about 5 to about 11, more preferably a pH in the range of about 6 to about 10, most preferably a pH in the range of about 7.5 to about 8.5. The treating solution includes a complexing agent capable of forming a complex with at least one metal ion. Preferred agents are capable of forming complexes with ions of, e.g., heavy metals, rare earth metals, actinides, and combinations thereof.
Examples of useful complexing agents include organic acids having more than one carboxyl group including citric acid, tartaric acid, oxalic acid, succinic acid, malonic acid, and ethylenediaminetetraacetic acid (xe2x80x9cEDTAxe2x80x9d).
Other useful complexing agents include lactic acid, sulphosalicylates, acetylacetonante, and azides (e.g., sodium azide).
The treating solution also includes ions, e.g., sodium ions, potassium ions, magnesium ions and combinations thereof. The treating solution is brought to the desired pH by the addition of an appropriate amount of buffer adjusting solution, e.g., base, which also provides the ions. Examples of useful bases include metal hydroxides including, e.g., sodium hydroxide, potassium hydroxide, calcium hydroxide, and magnesium hydroxide, and sodium azide. The sodium azide can function as both the complexing agent and a source of sodium ions.
The addition of ions can be used to convert substantially all of the particles in the medium to a single salt form, e.g., the sodium form, such that the medium exhibits an increased propensity to selectively sorb predetermined ions, e.g., cations or anions. Preferably ions are added to convert substantially all of the medium to the sodium salt form. Preferably the medium exhibits a propensity to selectively sorb strontium ions.
The treating solution is used to treat a medium that is capable of sorbing metal ions. The medium includes particles capable of removing ions from fluids through mechanisms such as, e.g., ion exchange (e.g., solid phase ion exchange), chelation, covalent bond formation, and sorption (e.g., adsorption, absorption and combinations thereof). Preferably the medium is capable of sorbing ions of radioactive particles, metals (e.g., heavy metals, rare earth metals, alkaline earth metals, and combinations thereof) and combinations thereof. Useful media sorb ions of metals from Groups IA, IIA, IB, IIB, IIIB, and VIII of the periodic table. Preferably the medium is capable of sorbing ions of metals such as, e.g., cesium strontium, silver, cobalt, chromium, gold, mercury, uranium, americium, plutonium, copper, iron, technetium, lead, zinc, and rhenium.
Typically the medium consists of finely divided, microporous particles. Preferably the particles have a relatively large area of active surface and a uniform size distribution. Useful particles have an average particle size in the range of about 1 xcexcm to about 100 xcexcm, preferably about 2 xcexcm to about 75 xcexcm, more preferably about 9 xcexcm to about 18 xcexcm. Suitable particles include inorganic, organic, and combinations thereof. Preferably the particles are ionically charged (e.g., cationic and anionic particles).
Useful inorganic media include metal titanates, where the metal is selected form Group IA and Group IIA metals (e.g., sodium titanate which includes nonatitanate), silicotitanates (e.g., crystalline silico titanate, and mixed salts of titanium silicates) and combinations thereof. Examples of commercially available inorganic particles include sodium titanates (available from Allied Signal Corp., Chicago, Ill.), crystalline silico titanates (available under the trade designation IONSIV from UOP of Tarrytown, N.Y.), sorbent particles available under the trade designation ATS from Engelhard Corporation, Iselin, N.J., and high capacity resins available under the trade designation NALCITE from Nalco Chemical Co., Naperville, Ill.
Examples of useful organic media include sulfonated styrene divinyl benzene resins (commercially available, e.g., under the trade designation CATEX from Sarasep Corp., Santa Clara, Calif.)), organic anion sorber (commercially available under the trade designation ANEX from Serasep), and organic cation sorber (commercially available under the trade designation DIPHONIX from Ichrome Industries of Chicago, Ill.).
Other useful commercially available particles include SAMMS self-assembled monolayers on mesoporous supports specific for mercury analytes having a formula SiO2xe2x80x94CH2CH2xe2x80x94SH (from Batelle Memorial Institute, Pacific Northwest National Labs, Richland, Wash.), and Clinoptolite.
The medium can also include derivatized particles. Useful derivatized particles include polymeric coated oxide particles and organic moieties covalently bonded to inorganic oxide particles. Derivatized particles are described, e.g., in U.S. Pat. Nos. 5,393,892 (Krakowiak), 5,334,326 (Bostick), 5,316,679 (Bruening), 5,273,660 (Bruening), and 5,244,856 (Bruening) and incorporated herein by reference.
The particles can be enmeshed in a variety of fibrous, nonwoven webs, which preferably are porous. Examples of such webs include polymer pulps, fibrillated polytetrafluoroethylene (PTFE), microfibrous webs, and macrofibrous webs. Examples of particle filled webs are described in U.S. Pat. Nos. 5,328,758 (Markell et al.), 5,071,610 (Hagen et al.), 5,082,720 (Hayes), and 3,971,373 (Braun), the disclosures of which are incorporated herein by reference. Other useful media may include those media described in U.S. Ser. No. 08/791,205 entitled, xe2x80x9cSpiral Wound Extraction Cartridge,xe2x80x9d which was filed on Feb. 13, 1997; U.S. Ser. No. 08/918,113 entitled, xe2x80x9cAbsorbent for Metal Ions and Method for Making and Using,xe2x80x9d which was filed on Aug. 27, 1997; and PCT publication WO96/29146 published Sep. 26, 1996 and incorporated herein by reference.
Another useful medium includes sponge-like (i.e., porous) medium prepared by compacting spray dried particles under low pressure (e.g., hand pressure) into a confined space and heating the compacted particles to a temperature of about 130xc2x0 C. for about 72 hours results. Such sponge-like media exhibit excellent separating ability and relatively low back pressure during use. Useful sponge-like media are described in U.S. Ser. No. 08/960,528 filed on Oct. 31, 1997 and incorporated herein by reference.
The medium can be in the form of an article such as, e.g., membranes (e.g., particle embedded membranes, and particle filled membranes), particle filled microfiber webs, particle coated filter paper, cartridges, columns (e.g., chromatography columns, short packed columns), disks and sheets.
The invention will now be described further by way of the following examples.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.